Frequency synthesizers are commonly used in wireless devices to generate sinusoidal output signals. A commonly used frequency synthesizer configuration is that of a phase-locked loop. Phase locked loop frequency synthesizers, for instance, are used to provide a local oscillator frequency within the wireless device.
A phase locked loop frequency synthesizer in accordance with the prior art is shown in FIG. 1. Phase locked loop frequency synthesizers generally contain a voltage-controlled oscillator (VCO) 101 combined with a phase detector 103. A signal of a given frequency (fREF) is applied to the phase detector 103. The output of the phase detector 103 is a voltage/current proportional to the phase difference between the input fREF and a second input coupled to the output of the VCO 101 through a divide by N frequency divider 105. The output of the phase detector 103 is provided to a loop filter 107, with the output from the loop filter 107 providing the input to the voltage controlled oscillator 101. A local oscillator frequency output 109 is provided from the voltage controlled oscillator 101. This output also provides the input to the divide by N frequency divider, thus completing the phase locked loop.
For devices that operate in the GHz range, an LC type of voltage controlled oscillator is commonly used. LC voltage controlled oscillators comprise a series of inductors and varactors that form an LC tank circuit. An example of a typical LC type voltage controlled oscillator is shown in FIG. 2. A pair of inductors 201, 202 and a pair of varactors 203, 204 are used in conjunction with a pair of transistors 207, 208 to build the LC voltage controlled oscillator. Positive feedback is created using the transistors 207, 208 to sustain the oscillation output.
The output oscillation frequency from the voltage controlled oscillator is varied by varying the input DC voltage Vc 210 across the varactors 203, 204. Vc is limited by the selected supply voltage, and thus the output frequency range of the voltage controlled oscillator is also limited by Vc. In many wireless applications, a wide frequency tuning range is desired to allow for multiple channels. Wide tuning range oscillators, however, require steep tuning curves, and this tends to create undesirable phase noise. In order to allow for a wide frequency tuning range without causing undesirable levels of phase noise, multi-band voltage controlled oscillators are used.
A multi-band voltage controlled oscillator adds a series of switching capacitors to the circuit shown in FIG. 2 in order to improve the output frequency range. Each switching capacitor can provide extra capacitance to the voltage controlled oscillator, thus increasing the output frequency tuning range of the oscillator.
FIG. 3A illustrates a multi-band voltage controlled oscillator. A series of switching capacitors 301, 302, 303, 304, 305, 306 can be connected to the voltage controlled oscillator by setting digital control voltages VK1, VK2, and VK3. Using the switching capacitors to select one of several bands allows a wide frequency tuning range, while avoiding an increase in phase noise that would occur in a single band oscillator with a steep tuning curve.
The tuning curves for a multi-band voltage controlled oscillator as shown in FIG. 3A are depicted in FIG. 3B. For each band, the frequency increases as Vc increases. An optimum usable range of tuning voltages is shown between VL and VH. When Vc exceeds VH (or when Vc is less than VL), the multi-band voltage controlled oscillator needs to change from one band to the next. When the tuning voltage exceeds the high-end limit, VH, for a particular band (typically determined by comparing the tuning voltage with a reference voltage equal to the high-end limit), one or more of the switching capacitors is switched off to jump the voltage controlled oscillator to the next higher band. Once the new band is selected, a waiting period is required to allow the phase locked loop to settle. If, after settling, the input voltage Vc remains outside of the predetermined limits, the process is repeated to select another band.
If a second input voltage comparison is undertaken before the minimum settling period for a particular phase-locked loop has expired, it can cause the phase-locked loop circuit to become unstable. Additionally, because the settling period varies depending on the characteristics of the phase-locked loop, the proper waiting time must be calculated for each individual phase-locked loop circuit configuration. This delay after band switching while the tuning voltage settles before a subsequent determination of whether the tuning voltage is within the predetermined voltage limits can be made is one disadvantage of prior art phase-locked loop circuits comprising multi-band voltage controlled oscillators.
The amount of delay required depends upon the individual dynamics of the phase-locked loop circuit. If a second input voltage comparison is undertaken before the minimum settling period for a particular phase-locked loop has expired, it can cause the phase-locked loop circuit to become unstable. Additionally, because the settling period varies depending on the characteristics of the phase-locked loop, the proper waiting time must be calculated for each individual phase-locked loop circuit configuration.
It is desirable to have a phase-locked loop circuit comprising a multi-band voltage controlled oscillator that is capable of automatic band selection without requiring a pre-calculated delay or settling period following band switching which is based on individual phase locked loop characteristics.